libxc: build fix with debugging disabled.

libxc: build fix with debugging disabled.

Currently hypercalls have only 5 arguments, hypercall->arg[0..4]. Do
not try and print arg[5] else:

        cc1: warnings being treated as errors
        xenctrl_osdep_ENOSYS.c: In function
        'ENOSYS_privcmd_hypercall':
        xenctrl_osdep_ENOSYS.c:30: error: array subscript is above
        array bounds

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>